Dietary Fiber and Testosterone: How Important is Fibre?

Dietary fiber is a generic term to refer to all plant-based dietary compounds that can’t be metabolized by the intestines. In short, it’s the ‘roughage’ that travels through your system intact. Looking pretty much the same as it enters your system (mouth), and as it exits the system (toilet).

By definition, fiber is either;

Insoluble fiber, which is the kind that won’t dissolve to water

Soluble fiber, which is the kind that does dissolve to water

Your body processes these fibers in multiple ways. For instance, certain soluble fibers can be converted into gelatin-like jelly, which is then converted into probiotics by the gut bacteria. And the kind of fiber that doesn’t get fermented, usually just acts as a bulking agent and pushes stuff out of the body on its way out (which is why high fiber intake can relieve constipation).

Certain fibers can also increase or decrease phytonutrient and micronutrient absorption from the foods we eat, and since some have claimed that dietary fiber can also modulate the rate at which the body removes steroid hormones, many have started believing that in order to improve your testosterone levels naturally, fibrous foods should be somewhat avoided, or generally fiber intake should be kept pretty low.

And another one where a change from high-fat low-fiber diet to a low-fat high-fiber kind resulted in a 12% drop in total testosterone levels with also 10% decrease in free-testosterone and DHT fell by 9%.

All the other kinds of studies that have examined fiber intake and testosterone levels, are studies comparing vegetarian/vegan diets to carnivore diets. This also makes the results inconclusive in terms of what is the effect of fiber solely, since plant-based diets come with significantly lower amounts of total dietary fat and are by definition correlated with lower testosterone levels (study, study).

Pretty much the only study that I managed to find that would of have looked the effects of fiber alone on hormones, is an in-vitro (test-tube) study which showed that various purified fibers were able to bind to steroid hormones in the following declining order of strength; flax -> wheat and oat brans -> corn brans -> oat hull. Theoretically this would support the claim that high-fiber intake increases the clearance rate of hormones, but since this is a study done inside test-tubes, who knows what happens inside the gut.

Conclusion

There isn’t any definitive proof to say that high-fiber intake would lower testosterone levels, since all the human studies on the topic are paired with low-fat intake or vegan/vegetarian test-subjects.

In one in-vitro study, purified and naturally occurring fibers were able to bind steroid hormones, which supports the claim that dietary fiber would increase the clearance rate of T, but then again this is a test-tube study and who knows what happens in living humans..

I would recommend sticking to the recommended ~30g fiber a day or less.

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